(1) Field of the Invention
The present invention relates to novel synthetically derived nanolayered silicate compositions with different structural units in adjacent nanolayers. In particular, the present invention relates to octahedral metal oxide nanolayers covalently linked to tetrahedral silicate nanolayers so that numerous silanol groups are present in the silicate nanolayers. The silanol groups are reactive to form derivative products particularly useful for removing heavy metals from contaminated water.
(2) Description of Related Art
The smectite family of layered silicate minerals, the so-called “swelling clays”, have theoretical surface areas of 700 meters square per gram or larger, depending on equivalent weight. However, accessing this surface area for important materials applications such as chemical catalysis, adsorption, and polymer-clay nanocomposite formation is not readily achievable, primarily because the elementary one-nanometer thick nanolayers stack one upon the another to form tactoids. This stacking behavior leading to tactoid (aggregate) formation is detected through the appearance of a 001 reflection in the X-ray powder pattern of the clay. Tactoid formation results in the mutual shielding of the basal plane surfaces of the majority of nanolayers, making the theoretical surface area generally inaccessible. For a typical smectite clay, such as montmorillonite, the surface area experimentally determined by nitrogen adsorption methods is limited to values in the range 1-50 square meters per gram. In order to achieve access to the interlayer regions of clay tactoids, the tactoids can be swelled by swelling solvent, thereby exposing the basal surfaces to potential guest molecules in the solution. However, the use of a swelling solvent to access the basal plane surface area is inconvenient and limits the applications to only swelling solvents.
Smectite clays can be pillared through the intercalation of robust cations such as in the gallery regions of the tactoids, but this approach to improving the available surface area also has severe limitations because the pillars themselves occupy the basal surfaces of the clay. Moreover, pillared clays have limited pore sizes pore sizes that exclude the adsorption of molecules with kinetic diameters above about 1.0 nanometer.
One promising approach to opening up the basal surfaces of smectite clays is to prevent the stacking of the nanolayers and avoiding tactoids formation. This relationship between nanolayers can be approximated by limiting the in-plane growth of the nanolayers in one direction so that the nanolayers are lath-shaped rather than disk-shaped, as is typical of most smectite clay minerals. Lath shaped nanolayers, such as those found for certain grades of synthetic laponite, (M. L. Occelli et al. J. Catal., 90, 256, 1984; J. Catal., 104, 331, 1987) tend to form edge-to-basal plane aggregates in a card house fashion, but the stacking of laponite nanolayers is never completely prevented. Another example of limited nanolayer stacking has been disclosed by Vogels et al. in patent WO 9607613. In this latter system, the stacking of synthetic saponite layers was limited from one nanolayer (that is, no nanolayers stacking) to not more than 20 nanolayers in a stacked tactoid. No explanation was provided for the limited stacking of nanolayers, but it is likely that the size of the nanolayers was limited in the in-plane directions, thus facilitating aggregation of the nanolayers in a card-house fashion.
The formation of nanolayered silicate phases in unstacked form holds great promise for applications in chemical catalysis, adsorption, polymer-nanocomposite formation, among many other materials applications. The usefulness of the such unstacked compositions would be greatly expanded if stacking could be prevented for nanolayers with an aspect ratio substantially greater than the aspect ratio of laponite (˜25) and more comparable to the aspect ratio of montmorillonite (>200). However, this is especially difficult to accomplish for large aspect ratio layered silicates because of the strong tendency of such large nanolayers to stack. A new process other than the card house mechanism is needed to prevent layer stacking for layered silicates with a large aspect ratio.